1. Field of the Invention
The invention relates to a method and apparatus for coating drum and flexible belt charge receptors for photocopiers. More particularly, the invention relates to an efficient method and modular apparatus for coating cylindrical or belt-like substrates by horizontally supporting the substrates in a planetary arrangement about a central horizontal axis and mounting a coating applicator in the center of the planetary array.
2. Description of Related Art
A photoreceptor is a cylindrical or belt-like substrate used in a xerographic apparatus. The photoreceptor substrate is coated with one or more layers of a photoconductive material, i.e., a material whose electrical conductivity changes upon illumination. In xerographic use, an electrical potential is applied across the photoconductive layer and then exposed to light from an image. The electrical potential of the photoconductive layer decays at the portions irradiated by the light from the image, leaving a distribution of electrostatic charge corresponding to the dark areas of the projected image. The electrostatic latent image is made visible by development with a suitable powder. Better control of the coating quality yields better imaging performance.
One method of coating substrates is to dip the substrate in a bath of the coating material. This method is disadvantageous because it usually results in a non-uniform coating. In particular, when the substrate is oriented vertically and dipped into a bath, the coating thickness tends to "thin" or decrease at the top of the substrate and "slump" or increase at the base of the substrate due to gravity induced flow of the coating material as the substrate is lifted from the bath. Thickness variations also occur even when the photoreceptor is oriented horizontally and dipped into the bath due to the formation of a meniscus as the substrate is removed from the bath. This variation in coating thickness causes variations in the performance of the photoreceptor. In addition, the dipping process requires additional processing controls because the bath must be constantly maintained in a state suitable for coating. The bath increases the size of the entire processing apparatus and is not readily adaptable to rapid changes in coating formulations. Further, changes in coating formulations are inhibited due to incompatibilities between formulations for successive coatings or layers. It is also difficult to incorporate cleaning and curing operations that are compatible with the dipping process for efficient modular operation as a manufacturing process.
In another method, an air assisted automatic spray gun uses high velocity air to atomize the coating formulation which is sprayed onto a substrate. Due to high mass transfer rates intrinsic to the use of atomizing air, this method entails considerable evaporative loss of solvent from the spray droplets and requires the use of slow evaporating solvents to prevent excessive solvent loss before the droplets arrive at the substrate. It is difficult to use this method in a sealed environment, and thus difficult to control the solvent humidity surrounding the substrates prior to, during, or after the coating process. In addition, the air atomized spray method creates a considerable amount of overspray which results in higher material usage. Air spray guns also are less advantageous for batch processing of a number of substrates.